Petroleum refining operations in which crude oil is processed to produce gasoline, diesel fuel, lubricants and so forth, frequently produce residual oils that have very little value. The value of residual oils can be substantially increased when processed in a "delayed coker unit". Residual oil, when processed in a delayed coker is heated in a furnace to a temperature sufficient to cause destructive distillation in which a substantial portion of the residual oil is converted, or "cracked" to usable hydrocarbon products and the remainder yields petroleum coke, a material composed mostly of carbon. A large vessel hereafter called a "coke drum" is provided at the furnace outlet to allow sufficient residence time for the hydrocarbons to complete destructive distillation reaction. The typical coke drum is a large, upright, cylindrical, metal vessel that may, for example, be in the order of approximately 90-100 feet in height (27.4-30.4 meters) and 20-30 feet in diameter (6.1-9.1 meters), although the actual structural size and shape of the coke drum can vary considerably from one installation to another.
Typically, a delayed coking unit has an even number of coke drums. The production of coke is a batch process. Coker feedstock is deposited as a hot liquid slurry in a coke drum. Lighter hydrocarbons which are products of destructive distillation flow out the top of the coke drum. Heavier material remains in the coke drum. When a coke drum is filled, residual oil from the furnace is diverted to another coke drum. The liquid mass remaining in the coke drum cools and is quenched as a part of the process. Solid coke formed as the drum cools must be removed from the drum so that the drum can be reused. While coke is being cooled in one or more drums and while the cooled coke is being extracted from one or more drums, other drums are employed to receive the continuous production of coke feedstock as a part of the delayed coker process.
Residual oil is heated to a temperature of typically about 900.degree. F. (477.4.degree. C.). The oil flows directly from the furnace to a coke drum. The liquid mass enters the drum, typically flowing through an opening in the bottom of the drum and, as the liquid level rises, the thermal cracking continues and layers of coke are laid down and solidify as the coke drum is cooled. Eventually the coke drum is filled substantially full with a solid mass.
When a coke drum is filled to the desired capacity, and after feedstock is diverted to another drum, steam is typically introduced into the drum to strip hydrocarbon vapors off of the solid material. The drum remains substantially full of coke that, as it cools, hardens into solid material.
It is a standard procedure to cool coke in a drum by the admission of steam then followed by water, that is, to cool the coke after the hydrocarbon vapors have been stripped off.
After a coke drum has been filled, stripped and then quenched so that the coke is in a solid state and the temperature is reduced to a reasonable level, quench water is drained from the drum through piping to allow for safe unheading of the drum. The bottom opening is uncovered, that is unheaded, to permit removing coke. Shot coke may have plugged off the drain line preventing a complete draining of the drum. Shot coke may also be loosely packed inside the drum and may "cave in" in an avalanche-like fashion and spilling onto the switch deck area below the coke drum causing substantial operating delay and creating potential hazards to personnel. Operating personnel are required to exercise reasonable caution to avoid coke hot water and hot vapors that may be released when a cave-in occurs. Procedures required to minimize the potentially harmful effects of a cave-in usually take a substantial amount of time and are not always completely effective. Once the unheading is complete, the coke in the drum is cut out of the drum by high pressure water jets. If the drum contains shot coke further avalanches my occur.
In some installations, a coke chute is located in a channel below the switch deck floor with a coke pit below it. Once the coke drum head is removed, the chute is raised to mate with the coke drum bottom flange. This process may not be completely satisfactory in that there is exposure to an avalanche of shot coke when raising the chute and the chute may be overwhelmed or may not function in the event of a cave in.
For all the above reasons, decoking a coke drum has been a relatively cautious and slow process especially when shot coke is produced and may expose workmen to a disagreeable and potentially dangerous environment. It is this situation to which the present invention is directed.
This invention provides improved safety when working around coke drums that substantially reduces the exposure of workmen to the hazardous conditions that may be associated with unheading and the initial steps of unloading a coke drum. It also benefits operations because it reduces the time required to safely return the coke drum back to service after removing the coke from the coke drum.
For background information relating to the basic concept of coke drums and the methods, system and processes by which coke is accumulated within a coke drum and removed therefrom, the following United States patents are helpful.
______________________________________ PATENT NO. INVENTOR TITLE ______________________________________ 1065081 Reubold Apparatus For Quenching Coke 3576263 Extensible Coal Bunker Construction 3611787 Aparatus For Minimizing Thermal Gradient In Test Specimens 3780888 Material Transfer Apparatus For A Rotary Drum 3917516 Coke-Cooling Apparatus 3936358 Method of Controlling The Feed Rate of Quench Water To A Coking Drum ln Response To The Internal Pressure Therein 3958700 Charging Machines 4135986 One-Spot Rotary Coke Quenching Car 4147594 One-Spot Cylindrical Coke Quenching Car and Quenching Method 4282068 Apparatus For The Transfer and Quenching of Coke 4284478 Apparatus For Quenching Hot Coke 4285772 Method and Apparatus For Handling and Dry Quenching Coke 4289585 Method and Apparatus For The Wet Quenching of Coke 4294663 Method For Operating A Coke Quench Tower Scrubber System 4312711 Fluid Cooled Quenching Cars 4344822 One-Spot Car Coke Quenching Method 4358343 Method For Quenching Coke 4396461 One-Spot Car Coke Quenching Process 4409067 Quenching Method and Apparatus 4437936 Process For Utilizing Waste Heat and For Obtaining Water Gas During The Cooiing of Incandescent Coke 4469557 Process For Calcining and Carbonizing Petroleum Coke 4512850 Process For Wet Quenching Of Coal-Coke 4557804 Coke Coolerr et al 4588479 Device For Cooling Incandescent Coke 4614567 Method and Apparatus For Selective After-Quenching Of Coke On A Coke Bench 4634500 Method of Quenching Heated Coke To Limit Coke Drum stress 4664750 Method For Coke Quenching Control 4726465 Coke Quenching Car 4743342 Coke Quenching Apparatus 4747913 Cooling Apparatus For Granular Coke Material 4772360 Thin Wall Coke Quenching Container 4802573 Process For Wet Quenching Of Coke 4832795 Coke Dry Cooling Chamber 4886580 Dry Quenching Coke Box 4988411 Filling Car for a Coke Oven Battery 4997527 Coke Handling and Dry Quenching Method 5024730 Control System For Delayed Coker 5098524 Coke Drum Unheading Device 5628603 Automated Chute System 5697408 Filling Containers ______________________________________
For reference to the system for controlling the discharge of coke from the open bottom end of a coke drum, U.S. Pat. No. 5,628,603 entitled "Automatic Chute System" is relevant.